Here is some very relevant thermal efficiency info from a wiki:
The second paragraph touches on the efficiency differences between gas and diesel engines......gasoline (otto cycle) engines suffer large frictional losses (throttling losses) at less than wide open throttle (WOT). Since diesels don't have throttles, they don't have these losses.
The frictional and pumping losses are losses in volumetric efficiency....
engines have a maximum thermal efficiency of about 25% to 30% when used to power a car. In other words, even when the engine is operating at its point of maximum thermal efficiency, of the total heat energy released by the gasoline
consumed, about 70-75% is rejected as heat without being turned into useful work, i.e. turning the crankshaft.
Approximately half of this rejected heat is carried away by the exhaust gases, and half passes through the cylinder walls or cylinder head into the engine cooling system, and is passed to the atmosphere via the cooling system radiator.
Some of the work generated is also lost as friction, noise, air turbulence, and work used to turn engine equipment and appliances such as water and oil pumps
and the electrical generator
, leaving only about 25-30% of the energy released by the fuel consumed available to move the vehicle.
At idle, the thermal efficiency is zero, since no usable work is being drawn from the engine. At low speeds, gasoline engines suffer efficiency losses at small throttle openings from the high turbulence and frictional (head) loss when the incoming air must fight its way around the nearly closed throttle; diesel engines do not suffer this loss because the incoming air is not throttled. At high speeds, efficiency in both types of engine is reduced by pumping and mechanical frictional losses, and the shorter period within which combustion has to take place. Engine efficiency peaks in most applications at around 75% of rated engine power, which is also the range of greatest engine torque (e.g. in most modern passenger automobile engines with a redline
of about 6,000 RPM, maximum torque is obtained at about 4,500 RPM
, and maximum engine power is obtained at about 6,000 RPM). At all other combinations of engine speed and torque, the thermal efficiency is less than this maximum.